The present invention relates to a power amplifier which may eliminate distortion and may have a high power efficiency.
In general, the power amplifiers are biased to Class A, B, C or D. In a transistor complementary push-pull circuit biased to Class-A operation, the emitter currents continuously flow through two transistors so that the output is not subjected to the distortion due to the switching of these transistors. However, with a power amplifier biased to Class-B operation, the emitter current flows only during the positive or negative half cycle of the load current and does not flow during the negative or positive half cycle so that the distortion occurs every time when the emitter current is turned on and off.
The Class-A power amplifiers are therefore advantageous over the Class-B power amplifiers in that no switching distortion occurs, but the maximum power loss of the former is about five times as much as that of the latter so that the Class-B power amplifiers are by far advantageous over the Class-A power amplifiers.
In a Class-D power amplifier a switching element is turned on and off by a square waveform pulse-width modulated with an input signal, and the output from the switching element is derived through a low-pass filter. Since the voltage and current are not simultaneously applied to the switching element, the power loss is almost negligible and consequently the power efficiency is very high. However, in general the Class-D power amplifiers are not provided with a negative feedback because of the provision of a low-pass filter for removing the ripple of a switching frequency so that the elimination of distortion by a negative feedback cannot be made.
As a result of the above comparison among the Class-A, --B and -D power amplifiers, it can be said that the less the distortion, the higher the power loss becomes, and it had been extremely difficult to provide a power amplifier with less distortion yet with a high power efficiency.